This invention relates to wireless communications systems, and more particularly, to wireless communications systems having variable power communications frames.
Some known communications systems may comprise communications cells in which transmitters may transmit information at a data rate that is selected from a plurality of fixed data rates that are available to the system. In such systems, information may be transmitted in communications frames that include a preamble portion. The preamble portion of communications frames may always be transmitted at a specific data rate. The preamble portion may be transmitted at the lowest one of the available data rates to accommodate receivers in the system that may not be capable of operating at the higher data rates that are available. Data rates for other portions of communications frames may be transmitted at any of the fixed data rates that are available in the system. Known techniques for implementing such systems have been deficient in meeting communications services demands (e.g., high speed throughput, low inter-cell interference, etc.) without substantially adding complexity, cost, circuitry, etc.
Such known systems include some wireless local areas network communications systems. Wireless local area network communications systems (xe2x80x9cwireless LANsxe2x80x9d) are typically used in applications that involve mobile computers, in applications where wireline installation is not feasible, etc. Such applications include warehouse inventory tracking, portable point of sale, shipping and receiving, package tracking, etc.
The IEEE 802.11 communications standard has been used by some vendors to provide interoperability between wireless LAN equipment. The 802.11 standard specifies a protocol in which information is transmitted in frames. The standard specifies features such as frame size, frame content information, data rates, roaming, etc. The primary type of information that was initially transmitted in systems that were designed to the 802.11 standard as published was information such as barcode information, point of sale information, package tracking information, etc. In such known systems, several remote terminals may be in communications with a single access point to receive and transmit information such as bar code information, point of sale information, package tracking information, etc. The standard as published specifies a communications medium that is shared by transmitters (e.g., an access point and one or more remote terminals).
In accordance with the principles of the present invention, variable power level communications for wireless communications systems may be provided. A wireless communications system, such as a wireless LAN, may be provided. Transmitters (e.g., remote terminals) and receivers (e.g., access points) in the communications system may communicate using communications frames. Communications services may be provided for a substantially contiguous geographic area with a plurality of transmitters and receivers in a plurality of cells. Communications frames may be transmitted in two substantially contiguous portions that are at two power levels. A first portion of a communications frame may be transmitted at one data rate (e.g., the lowest data rate supported by the system) and at one power level. The remaining portion of a communications frame may be transmitted at one data rate (e.g., other supported data rates) and at another power level. The power level for the remainder portion may be greater than the power level for the first portion. The first portion may be a preamble portion of a communications frame. A plurality of cells and a plurality of channels may be used. Each cell may be assigned one channel from the plurality of channels. The number of cells may be greater than the number of available channels. Positions of cells in a communications system may be determined based on the variable power levels of the communications frames, number of channels, and the data rates. A power level for a first portion that is at a lowest data rate may be selected so that the first portion may have approximately the same signal propagation characteristics as a remainder portion that is at the next lowest data rate at another power level (e.g., higher power level). Transmitters may be configured to operate using a spread spectrum communications technique, such as direct sequence spread spectrum techniques. Transmitters may listen to a channel or frequency band on which the transmitter is configured to transmit and may determine whether the channel is available for transmission.